tst_qgraphicsproxywidget: skip crashing test on Mac/Windows.

Created QTBUG-33067 for tracking the issue, after some debugging.

Change-Id: Iaf5556db2e0858e40a7cf6c9dbbe7e6fd6120bac
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Skip tst_QGraphicsProxyWidget::deleteProxyForChildWidget on Win

This tests crashes a lot lately.

Task-number: QTBUG-29684

Change-Id: I6892238dc071f050b0208dd5b4843629fa707347
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>